Our AC failed in the summer. A local contractor diagnosed the problem as with the blower motor. He replaced it, and the system works. But, he and we noticed a hum, a low resonance sound, coming from the vents. He said he would fix it. But after repeated calls, which he fails to return, I think he is gone. What could I do, if anything, to lessen the resonance or hum? Is my only option getting another blower motor and installing it?